Influenza—Introduction
Influenza is a highly contagious respiratory disease caused by the influenza virus. Influenza respiratory infections, and particularly influenza A because of its continuous antigenic evolution, have the potential to lead to deadly pandemics (e.g., 1918, 1957 and 1968). In recent history, events such as those of Hong Kong in 1997, SARS in 2003, as well as the transmission of H5N1 avian influenza directly to humans, underscore the scope and severity of the consequences associated to such infections. In fact, according to the WHO there are 3 to 5 million severe cases of influenza each year and 250 000 to 500 000 deaths from the virus.
The influenza virus, which belongs to the orthomyxoviridae family, is enveloped and contains 8 single-stranded RNA segments coding for 10 to 12 proteins. While there are three types of influenza viruses (A, B and C), only types A and B cause significant illnesses in humans, type A viruses being the most problematic. Type A influenza viruses are responsible for most of the seasonal epidemics and are further subtyped according to surface glycoproteins: hemagglutinin (HA) and neuraminidase (NA). The influenza virus evades the host immune system by undergoing a continuous antigenic evolution through the processes called antigenic <<drift>> and <<shift>>. Antigenic drift is the evolution of viral strains through frequent mutations among antibody binding sites of surface antigens leading to the emergence of new variants not adequately recognized by the host immune system. Antigenic drift is the reason why, every season, it is necessary to identify and predict the most probable strains that will circulate in order to produce the most appropriate vaccines for annual vaccinations. Antigenic shift results from the reassortment of the genetic material of co-circulating strains, leading to the replacement of surface glycoproteins HA, and less frequently NA, which in turn leads to the emergence of severe epidemics and even pandemics (1). Such a scenario is possible for the H5N1 avian virus which has a 50-60% death rate and the H5 component of which has never circulated in the human population. Because new viruses generated through antigenic shift also undergo antigenic drift, this means that in the case of the H5N1 virus, this could lead to efficient transmission from human to human and thus to a pandemic.
Unlike surface antigens, the internal proteins of the virus do not sustain the same mutational pressures and remain more conserved between strains. During the host adaptive cellular immune response, CD8 T cells eliminate infected epithelial cells through a process that involves perforin, granzymes, and cytokines such as TNF-α and IFNγ. Although it does not provide sterilizing immunity, as does the humoral response, the cellular response significantly reduces lung viral titers. In addition, as opposed to the humoral response which is ineffective against viruses bearing mutated surface antigens, the cellular response recognizes internal epitopes which tend to be conserved between viral strains. It has been shown both experimentally and clinically that the influenza virus easily evades the host humoral response to its surface antigens thanks to its particular mutational characteristics.
Two classes of anti-influenza drugs are currently available: inhibitors of the viral M2 channel (e.g., amantadine and rimantadine) and inhibitors of the viral neuraminidase (e.g., zanamivir and oseltamivir) (2). Inhibitors of the viral M2 channel interact directly with the viral M2 ionic channel which participates in the acidification and the decapsulation of the virus in cellular endosomes, and the viral neuraminidase allow the detachment of nascent virions. Targeting viral proteins has proven to be an effective strategy. However, because of the mutational characteristics of the influenza virus and the widespread use of antiviral drugs, resistance has become an important problem (2, 3). As a result, inhibitors of the M2 ion channel are no longer recommended for the prophylactic treatment of influenza. Furthermore, according to the Center for Disease Control, almost all currently circulating strains of H3N2 influenza A are resistant to amantadine and almost all circulating H1N1 influenza A seasonal strains are resistant to oseltamivir (Tamiflu™). Fortunately, most H1N1 influenza A strains stemming from the 2009 pandemic are susceptible to the drug, although several resistant strains of this virus have been isolated. Therefore, targeting host cellular mechanisms that are crucial for influenza viral entry, protein synthesis, maturation, and replication, provides an exciting alternative strategy potentially obviating the resistance problem.
Hemagglutinin (HA) protein plays an essential role in binding to and entering into host cells during the virus infection process. Hemagglutinin (HA) binds to monosaccharide sialic acids that are present on the surface of its target host cells. The cell membrane then engulfs the virus through endocytosis and forms endosomes. The binding affinity of a type of influenza virus to sialic acids on epithelial cells of the respiratory system, typically in the nose, pharynx, trachea, bronchi, bronchioles, alveoli and lungs of mammals and intestines of birds, can affect the capability of the virus to infect the species and the capability to spread among different individuals.
Influenza HA is synthesized as a single protein precursor termed HA0 and since the virus does not encode any protease, host cell proteases are required for the cleavage of HA0 into subunits HA1 and HA2. This cleavage is required for the protein to change conformation in the acidic conditions in the endosome (9, 10). This change in the protein's conformation exposes the hydrophobic fusion peptide located in the HA2 subunit (11, 12). This allows the virus to fuse with the host cell. The hemagglutinin proteins of pathogenic avian influenza viruses are characterized by multibasic cleavage sites containing furin-like recognition sequences RXXR (13, 14). Since some subtilisin-like proteases such as furin or other proprotein convertases are ubiquitous, the HA glycoprotein of avian viruses utilizes multiple tissues and sites for its activation and allows infection and replication of these viruses in many cell types (pantropicity) (14). One of the severe manifestations of avian flu virus is a life-threatening encephalitis. On the other hand, the HA glycoprotein of non-avian viruses does not have the polybasic furin-recognition site. These viruses have monobasic cleavage sites recognized by other proteases (e.g., TTSPs) of the host (14).
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.